Getting network error 413 Request Entity Too Large event with size limit settled to 50mb and correct payload size

I'm getting network error 413 Request Entity Too Large, even when the configured size limit is 50mb and the payload size is a lot of shorter than that.

Status Code: 413 Request Entity Too Large

response headers:

Connection: close
Content-Length: 585
Content-Type: text/html
Date: Tue, 19 Oct 2021 15:36:06 GMT
Server: nginx/1.20.0

request headers:

Accept: application/json, text/javascript, */*; q=0.01
Accept-Encoding: gzip, deflate
Accept-Language: en,es-ES;q=0.9,es;q=0.8
Connection: keep-alive
Content-Length: 2072572
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Cookie: ...
Host: ...
Origin: ...Referer: http://devxd.us-east-1.elasticbeanstalk.com/admin/post/edit/22/
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/94.0.4606.81 Safari/537.36
X-Requested-With: XMLHttpRequest

in my nginx.config file i have:

    files:
   /etc/nginx/conf.d/proxy.conf:
     owner: root
     group: root
     mode: "000644"
     content: |
       client_max_body_size 50M;

       # Elastic Beanstalk Managed
     ...

the curious thing is that this config is working fine in prod